Memristor operation is strongly related to the stoichiometry of the device active layer. These devices operate in a small range of stoichiometries within which device performance is optimized. Unfortunately, the range of stoichiometries that yield operable devices is hard to reproducibly achieve using standard flow-control deposition in reactive sputtering (the most technologically relevant approach), and deposition parameters unpredictably change as the surface conditions inside the deposition chamber vary over time.
A major class of resistive memory devices (memristors, ReRAM, etc.) is based on transition metal oxide (TMO) films. In these films, mobile oxygen vacancies allow memory devices to exhibit multiple resistance states. The TMO stoichiometry determines the mobile oxygen vacancies, and thus the performance, of these devices. Reactive sputter deposition (RSD) is the most technologically relevant TMO deposition process. A major difficulty with RSD is that the optimum TMO stoichiometry occurs at an unstable operating point of the RSD tool.
Relevant TMO devices include Ta2O5-based devices, which have recently demonstrated impressive endurance and forming-free results. Deposition of sub-stoichiometric Ta2Ox (e.g., where x<5) films is a critical process in order to produce the required oxygen vacancies in some of these devices. Accordingly, there is a need for methods and apparatuses that provide reproducible films having controlled and optimized composition and/or stoichiometry.